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Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar) are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2H/δ18Osource-water), which usually reflects mean annual precipitation (δ2H/δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation. Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests, as well as grassland soils along a Central European transect in order to investigate the variability and robustness of various proxies, and to identify effects related to vegetation. Soil pH-values derived from brGDGTs correlate reasonably well with measured soil pH-values, but systematically overestimate them (ΔpH = 0.6 ± 0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (∆TMA = 0.5 °C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar/precipitation) is lower for grassland sites than for forest sites due to "signal damping", i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ~-21 ‰ ± 22 and Δδ18O = ~-2.9 ‰ ± 2.8). Similarly, relative humidity during daytime and vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV = ~-17 ± 12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can be likely explained by understory grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoil. The large uncertainty likely reflect the fact that biosynthetic fractionation is not constant, as well as microclimate variability. Overall, GDGTs and the coupled δ2Hn-alkane-δ18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Long-chain n-alkanes became increasingly used for paleoenvironmental studies during the last years as they have the great potential to reconstruct past changes in vegetation and climate. They mostly originate from leaf waxes of higher terrestrial plants, are relatively resistant against physical and chemical degradation and can thus serve as valuable biomarkers that are preserved in various sedimentary archives for at least millennial timescales. However, before any robust interpretation of the long-chain n-alkane patterns in sedimentary archives, reference samples from modern vegetation and topsoil material should be investigated at a regional scale. Apart from Central and South-Eastern Europe, such systematic regional studies on modern plant and topsoil material are still largely lacking. To test the potential of leaf wax derived n-alkane patterns for paleoenvironmental studies in the semi-humid to semi-arid southern Caucasus region, we investigated the influence of different vegetation types on the leaf wax n-alkane signal in modern plants and topsoil material (0–5 cm) from eastern Georgia. We sampled (i) sites with grassland that included steppe, cultivated grassland and meadows, and (ii) sites that are dominated by deciduous hornbeam forests. The n-alkane results show distinct and systematic differences between samples from sites with the different vegetation types: n-alkanes derived from sites with grassland are mainly dominated by C31, while n-alkanes derived from sites with deciduous trees show high abundances of C29. Thus, chain-length ratios allow to discriminate between these two different vegetation types and have a great potential when used for regional paleoenvironmental reconstructions. As degradation of organic matter can affect the leaf wax n-alkane distribution, we further present an updated end-member model that includes our results, accounts for degradation effects and enables semi-quantitative reconstruc-tions of past vegetation changes in the southern Caucasus region.